The proposed NRSA project will be part of a research training plan to prepare the P.I. for a career as a clinical investigator of the chronobiological and psychological mechanisms of mental health. This NRSA project will be the first study to integrate theory and expertise from leading chronobiological and cognitive-behavioral SAD researchers to test an integrative model of seasonal affective disorder (SAD). This study will use a prospective, longitudinal design to clarify the etiological underpinnings of SAD in a test of Rohan's integrative cognitive-behavioral model of SAD by (1) comparing the vulnerabilities between SAD patients and never-depressed matched controls at summer and winter, and (2) testing the relative and combined contributions of the chronobiological and cognitive vulnerabilities in predicting winter depression severity in patients and controls. The measures selected for the cognitive vulnerability are established measures of cognitive vulnerability to Nonseasonal depression that have research to support their relevance to SAD: brooding rumination; cognitive reactivity, and a measure of the SAD- specific seasonal cognitions proposed in Rohan's model. The chronobiological vulnerability will be measured as seasonal change in Phase Angle Difference (PAD): the difference in timing between circadian phase (as marked by the Dim Light Melatonin Onset) and the averaged midpoint of sleep (as recorded by wrist actigraphy) for each participant. To test between-group effects and the longitudinal prediction of winter depression severity, all measures will be completed once in the summer when the SAD patients are relatively asymptomatic, and once in the winter when the SAD patients are experiencing a Major depressive Episode, per DSM-IV-TR criteria. The summer cognitive measures and a seasonal change in PAD will be utilized as statistical predictors of winter depression severity in a longitudinal linear mixed model analysis. To control for baseline depression severity, depression severity measures will be obtained at both the summer and winter laboratory visits, with the summer measure will be entered into the analysis as a control variable. These cognitive and chronobiological predictors will be examined for unique and interactive effects on winter depression severity. As a collaboration of the leading disciplines in SAD research, this study represents the first test of an integrative etiological model of SAD recurrence. An integrative etiological model holds clinical implications for the treatment of SAD. Specifically results may inform whether new treatment strategies, i.e. the development of novel intervention strategies targeting these vulnerabilities and/or utilization of a treatment hierarchy, are warranted to reduce annual SAD recurrence.